Gone are the days when to become good at cycling you just rode your bike.  Now we have cycle computers, heart rate monitors, GPS and power monitors to tell us to ride harder, slower or to give up and go home!  Apart from the immense amount of entertainment these toys provide they do actually have a purpose and that is to give you an indication of how hard you are working.

Twenty years ago cycling computers were the gadget to have, then heart rate monitors came along and now it's surprising to see a cyclist without one.  However, now we have power output that has actually been around for over a decade but hasn't become very well used until the last few years.  But what is power output?

Simply, power is energy per unit of time or the rate of doing work.  Power is measured in watts and producing one watt is equivalent to expending one joule of energy per second (or 0.01433 kilocalories per minute).  Accurate training program prescription relies on setting intensities for training efforts whether they are low, high or in between.  This can be done via heart rate, cycling speed, by a rating of perceived effort or by using power output.  Most people are familiar with heart rate or cycling speed, but less so with power output.  Using power output has several advantages over heart rate or cycling speed.  This is because it is a constant measure and does not rely on your internal physiology (heart rate) or environment (cycling speed).  As an example, when we become dehydrated we experience what is called cardiac drift.  Despite riding at the same cycling speed or power output our heart rate rises, therefore making heart rate training zones invalid.  However, power will tell you exactly what is happening no matter your hydration status or what the weather is like.  You can compare day to day training and racing week to week and so on.

As an example, there is some data from a rider who completed two Saturday morning Auckland Waterfront 16km time trials:

Based on time and average speed you would instantly think they rode better in time trial two and that their training was working.  But in actual fact they sustained the same power output in each time trial so they have not improved.  The large difference in time came from improved body position, reducing drag, equipment aerodynamics and changes in weather.

A valid and reliable measure of power used to be available only in sports science laboratories, however over the last decade several power output monitors have become available.  It is extremely difficult to accurately quantify and measure power output therefore they are not cheap.  Each model has its advantages and disadvantages (eg. Some only use shimano components) so when looking to buy be aware of their constraints and compare this with what you are hoping to achieve instead of just looking at the price tag.

1. Ergomo Sport – Measures power through torque in the bottom bracket – from one leg only.
2. Polar – Calculates power through a chain vibration sensor that measures chain tension.
3. PowerTap – Has a torque measuring hub in the rear wheel.
4. SRM – Measures torsion in the crank arms or essentially hom much push you are putting into the pedals.

The benefits of training with power are enormous as it drastically improves the specificity of your training, and this is why the professional cyclists are using it.

Amy Taylor is one of the founders of Kinetic Edge – Training Technology (www.kecycling.com).  Amy is a Auckland based Exercise Physiologist with an MSc(Hons) in Sports Science.  She has been cycling for 14yrs and coaches recreational to world champion cyclists.

This article originally appeared in the December issue of Endurance Sport Magazine Issue 21.